Persistent organic pollutants (POPs) comprise a group of chemicals that degrade slowly in the environment, bioaccumulate and have toxic properties. Many are semi-volatile and thus subject to long-range atmospheric transport and exchange among media (air, water, soil, vegetation). Various initiatives have arisen to control or eliminate POPs on regional and global scales, the most recent and widely publicized being the Global POPs Protocol, signed in Johannesburg in December 2000. Signatories of the protocol agreed to a global ban of 12 chemicals, which included organochlorine pesticides, industrial chemicals and products of incomplete combustion (Table 1). Canada is also a signatory to the UN ECE POPs protocol which lists 16 chemicals (the UNEP 12 plus 4 others) for a phase-out by countries of western Europe including Russia, Canada and the U.S.A. (Table 1).

Table 1: Current UN ECE and UNEP POPs list

Mercury on the other hand is a natural element. In many ways mercury behaves like a semi-volatile organic compound. It occurs in small amounts in coal and other fossil fuels, in municipal garbage, notably from electrical apparatus, and in sulphide ores. When municipal waste, coal or other fuels are burned, or when ores are smelted, mercury is driven off as a gas to the atmosphere. Once in the atmosphere, it can move thousands of kilometres with moving air masses before being oxidized to a charged species and scavenged by precipitation. Mercury is of concern because in its organic form as methyl mercury it is a neurotoxin. As a signatory to the UN ECE LRTAP protocol on heavy metals, Canada is committed to reduce mercury emissions from industrial and waste sources.

The by-products of combustion on the POPs list include PAHs (polycyclic aromatic hydrocarbons), and polychlorinated dioxin/furans. Canada has committed to reduce emissions of these compounds under the UN ECE protocol. Like mercury, the PAHs and PCDD/Fs have anthropogenic and natural sources. The primary anthropogenic source of PAHs is from combustion of carbon-based fuels that are burned with insufficient oxygen. Natural sources like fossil fuels and combustion of wood and peat have existed for millions of years. PAHs generally do not have a high acute toxicity but several of them are potent carcinogens. The PCDD/Fs are by-products of synthesis of many chlorinated organic chemicals, e.g., chlorophenols, and in chlorine bleaching of wood pulp, however, these sources have been greatly reduced during the 1990s.

Substantial reservoirs exist for many POPs in the Canadian environment due to historical usage stretching back to the 1930s and 1940s. For example, organochlorine pesticides contaminate soils in agricultural regions, sediments in remote lakes sprayed for insect control, and in the urban environments treated for insect pests. PCBs are present in urban and rural soils, landfills and contaminated industrial sites. The same situation exists in most developed countries in the northern hemisphere. Also, Russia continues to use PCBs for electrical equipment. Limited usage of DDT for malaria control is still permitted under the Global Protocol. Other OC pesticides are still applied in Mexico, Central American, African and Asian countries. Mobilization from these reservoirs will continue to supply POPs to the atmosphere on a regional and hemispheric scale, even after current usage is stopped.

Atmospheric transport and deposition is a major pathway of contamination of aquatic environments by POPs and mercury in Canada. POPs and mercury (Hg) are routinely found in air and precipitation throughout Canada based on measurement programs in the Great Lakes region, the Rocky Mountains, and in the Arctic. Thus water quality concerns for these chemicals must include consideration of the input from and the escape to the atmosphere.

Mercury is transported through the atmosphere almost entirely as gaseous, elemental mercury. As such, it is only slowly removed by rain and snow and so has a low rate of deposition. Recent research in the Canadian Arctic has shown deposition of mercury occurs as polar sunrise releases bromine from sea salts where ionic bromine has accumulated over the winter. Thus, the unique photochemistry of the Arctic conspires to produce a form of mercury, which is quickly deposited from the atmosphere onto the snow pack. A critical--and so far unexplored--link is transfer of the deposited mercury to the food chain.

Concentrations of PCBs, DDT and other persistent organochlorine pesticides remain high in many aquatic food webs in Canada as a result of emissions from old sources and from long-range transport and deposition. In the Great Lakes region, levels of PCBs and DDT have declined significantly in top predators. For example, continued decline in PCBs in herring gull eggs reflects lower emissions following controls on open uses. Declines in Great Lakes lake trout and walleye have not been as dramatic especially since the mid-1980s reflecting continued emissions from urban areas and recycling of contaminants within the lakes (Pierce et al. 1998). There are very limited time-trend data for POPs in fish outside of the Great Lakes. In the Arctic, significant declines of PCBs and DDT have been observed in seabirds (Braune et al. 2001) but the story is mixed in the case of marine mammals. Significant declines of DDT have been found in ringed seals since the early 1970s (Addison and Smith 1998) but not for PCBs in blubber of beluga from southeast Baffin Island over the period 1982 to 1996 (Stern and Addison 1999).

Despite the declining levels, the interim guideline for PCB of 0.32 ng TEQ/g ww, designed to protect Canadian wildlife that consume fish and shellfish (Environment Canada 1998) is routinely exceeded by both predator and forage fish in many areas. Also, PCB levels in surface waters in many small rivers draining urban areas regularly exceed the 1 ng/L EQG. In the case of methyl Hg, the interim guideline of Hg of 22 ng/g ww for protection of fish consuming wildlife is exceeded in almost all fish measured to date in Canada. In Ontario, 95% of fish consumption advisories in lakes are due to Hg with the remainder due to PCBs and toxaphene (OMEE 1997). In Kejimkujik National Park in Nova Scotia loons' elevated levels of Hg have been associated with reproductive effects (Evers et al. 1998).

Experience from the Great Lakes and the Arctic suggests that for communities with high fish consumption, such as Aboriginal communities on remote Canadian Shield lakes, even relatively low levels of POPs and Hg will result in exceedances of Tolerable Daily Intakes (Jensen et al. 1997). Numerous studies of fish consumers in the Great Lakes region (recently summarized in Johnson et al. 1998) suggest that exposure to contaminants via high rates of fish consumption causes disturbances in reproductive parameters and neurobehavioural and developmental deficits in newborns and older children. There are insufficient data with which to assess if people in other regions of Canada who also rely on country foods, especially fishes, will exceed TDIs. With the exception of programs run by the Ontario Ministry of the Environment there are no current provincial or federal contaminants survey programs that address this information gap.

Levels of mercury, unlike the PCBs and DDT, have increased in the past 20 years in fish-eating birds and mammals. A striking example is the twofold increase from 1975 to 1995 observed in mercury in thick-billed murre eggs in the Canadian High Arctic (Braune et al. 2001). Another, independent line of evidence that mercury inputs to remote locations have been increasing comes from profiles in sediment cores. These also infer that inputs have increased greatly relative to pre-industrial times.

Emerging Issues

With the development of regional and global agreements to ban POPs, the attention of regulators and researchers has turned to other chemicals with similar properties. The UNEP and UN ECE lists are not closed. New substances can be added if they meet the scientific criteria for inclusion and if ratified by the signing parties. The Evaluation Division of CCEB has recently prepared a list of 100 chemicals in use in Canada that, in the opinion of an expert panel, may meet criteria for persistence, toxicity and in some cases bioaccumulation (Table 2). The criteria for inclusion are based on Canada's Toxic Substances Management Policy (Environment Canada 1995) and include evidence for transport to remote regions, long persistence in water, soil or sediment, octanol-water partition coefficients >5000 and potential toxicity.

Table 2. EC-CCEB listing of top 100 potential POPs in current use in Canada

Chemical Class

Number in Class

Number P. B. & T.*

Examples

Pigments & dyes

16

13

halo-, methoxy substitutedPAHs and heterocycles

Chlorinated substances

13

3

lindane, atrazine, PCP

Antioxidants

7

5

thiol- and amino-substituted benzenes

Nitro substances

1

1

trifluralin

Brominated flame retardants

16

9

BDPEs, brominanted cycloalkanes

Musks

6

5

musk ketone, musk xylene, poly-cyclic musks

Phthalates

7

0

clohexyl, nonyl, tridecyl

Miscellaneous organics

27

18

terphenyl, imidazoles. S- and N-substituted biphenyls

Perfluorinated carboxylates

4

4

PFOA, PFDA

Perfluorinated sulfonates

3

3

PFOS, PFOSamide

Total

100

61

*Number of compounds that may be persistent, bioaccumulative and toxic.

With the exception of two or three pesticides on the list in Table 2, there is very limited information available on the physical properties, half-lives of persistence or bioaccumulation of these compounds, or on current environmental levels. This is a major problem with efforts to identify and measure chemicals other than those on the current POPs list. While there are environmental measurement data on some pesticides listed in Table 2, as well as phthalates, musks, and brominated flame retardants, the vast majority of the 100 compounds remain unstudied in Canada and elsewhere.

Studies on brominated flame retardants (BFRs), perfluorocarboxylates and sulfonates have recently begun in Canada with funding from the Toxic Substances Research Initiative. The work on BFRs has shown that polybrominated diphenyl ethers (PBDEs), additives used in the manufacturing of plastics, paints, textiles and electrical devices, are increasing rapidly in the environment. For example, temporal trend data for PBDEs in gull eggs from Snake Island in Lake Ontario show an increase of 65- fold between 1981 and 1999; and lake trout from Lake Ontario showed a 300-fold increase in PBDEs between 1978 and 1998.

The presence of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in liver samples of top predators, such as polar bears and fish-eating birds, as well as in human blood samples, was recently reported by the manufacturer, 3M Company. These compounds are anions and have low octanol-water partition coefficients. However, they persist in biota because of the great stability of the perfluoro group and because they are recycled in the entero-hepatic circulation. This new bioaccumulation pathway may be important for a wide range of chemically stable, polar compounds, and this raises the issue of whether criteria for identifying POPs will have to be modified.

Not included on the list are two groups of chemicals, the polychlorinated naphthalenes (PCNs) and chlorinated paraffins which resemble current POPs in terms of physical properties and molecular structure. PCNs were first manufactured for industrial purposes in the early 1900s until 1977 in the U.S.A. They are of concern because several congeners elicit dioxin-type responses similar to co-planar PCBs. PCNs are also a by-product of combustion and analyses of dated sediment cores show increases of combustion- related PCNs in recent layers. Combustion-derived PCNs are also evident in air samples from the Toronto area and the Great Lakes. Chlorinated paraffins are another complex group of chemicals with physical properties similar to many chlorinated pesticides. Short-chain chlorinated paraffins (SCCPs) have been proposed for virtual elimination in Canada under CEPA Track 1. SCCPs are detectable in the Canadian environment including remote lake sediments and marine biota in the Arctic. However, highest levels are found in biota and sediments near urban areas. Like the PCNs, SCCP concentrations in the environment appear to be declining based on profiles in dated sediment cores.

Recent studies have found that halogenated phenolic compounds (HPCs), including pentachlorophenol (PCP), hydroxy-substituted PCBs (OH-PCBs) and hydroxy-brominated diphenyl ethers are present in the blood of marine mammals and humans in Canada from the Arctic and temperate regions of North America (Sandau et al. 2000). HPCs have been shown to have endocrine disruption potential based on in vivo and in vitro assays (Kester 2000; Schuur et al. 1998), while OH-PCB sources may be primarily from metabolism of other PCB sources.

Several persistent current use pesticides (PCUPs) such as those identified in Table 2 also have characteristics which lead to persistence and long-range transport. Endosulfan and lindane can be detected in freshwater and marine biota throughout Canada even in sites far removed from use areas. In general, there is no monitoring of other potential PCUPs in fish. Additional possible candidates are dacthal, pentachlorophenol, trifluralin and related dinitroanilines. Other studies have shown that the herbicide trifluralin is detectable in arctic snow and that dacthal, widely used as a fungicide in the U.S.A., is detectable in air at Alert in the High Arctic and at Tagish, Yukon, with highest levels during the summer months (D. Muir, unpublished data, NWRI, Burlington, ON).

Data are needed on the toxicology of individual compounds of POPs, e.g., bioaccumulative components of toxaphene. Current lack of data makes risk assessment of actual environmental residue problematic. For example, Health Canada has assigned large safety factors for TDIs for chlordane and toxaphene due to lack of data--we may be overstating the concern for chlordane and toxaphene contamination as a result.

Basic information is needed on the mammalian, avian and aquatic-toxicology of new POPs, i.e., typically limited to LD50 in one species.

Basic physical property information is needed on "new" POPs such as those on the EC-CCEB list, especially at temperatures relevant to Canada, in order to predict environmental fate and bioaccumulation potential. Presently, Kow values are only available based on structure activity. Biodegradation, photolysis and hydrolysis data, which are needed to predict overall environmental persistence and LRTAP potential, are lacking for most new POPs.

Modelling capability for forecasting the trends in environmental levels and fate of old and new POPs needs to be refined. This may require new approaches for modelling as well as physical-chemical property data.

Analytical methods information is needed for new POPs. For example, published analytical methods are unavailable for 80% of the compounds on the EC-CCEB top 100 POPs list. Method development is limited by lack of facilities such as clean rooms to limit contamination from products in current use as well as lack of appropriate instrumentation.

There is limited knowledge available on current levels and trends of old and new or potential POPs in all environmental compartments, especially aquatic and terrestrial. This makes exposure and risk assessment for new chemical exposures difficult--relying solely on Predicted Environmental Concentrations (PECs) that are based on limited phys-property data.

The lack of knowledge of anthropogenic versus natural sources of mercury, and their relative biological availability, continues to be a problem in the assessment of mercury in the Canadian environment. The biological implications of total mercury concentrations in biota need further investigation. Current assessment methods assume all mercury can be converted to methyl mercury. The extent of conversion of various forms of mercury, i.e., selenides, inorganic mercury, organic species, to methyl mercury is unknown.

Detailed investigations of the mercury biogeochemical cycle are needed to understand the transfer of mercury from the atmosphere to aquatic food webs. Also unknown is the reverse cycle of mercury: how much is reduced to elemental mercury in snow and on terrestrial surfaces and volatilized again?

PAHs represent a major contaminant group in Canadian surface water but much more information is needed on PAH levels in biota where PAHs are present as epoxides and hydroxylated derivatives. In general, there is a need to develop a better understanding of these by-products in the environment.

Assessment of time trends in POPs resulting from UN ECE and UNEP agreements is limited by lack of commitment to long-term research and monitoring programs. At present there are long-term air and precipitation programs (MSC/IADN) and one on fish (Great Lakes, DFO) and one on fish-eating birds (CWS). There are other smaller ad hoc efforts by individual scientists for temporal trends using biota or sediment cores, e.g., funded by NCP, TSRI or NREI. There are no long-term trends measurements for POPs or new POPs in surface waters.

The knowledge gaps identified for POPs and mercury need to be addressed.

A research and monitoring program on POPs with a national focus is needed that ensures that information on current levels and time trends of old and emerging POPs are available in all regions in major environmental media, e.g., water, air, sediments and fish.

Development of capacity (instruments, clean room facilities, trained personnel, etc.) to measure new POPs and PCUPs in environmental samples within Environment Canada and within other government and university labs.

The capacity to link the chemical measurements to biological effects needs to be strengthened through additional funding for long-term field studies, and support for training.